False positive for `substr()`

[jrmajor]

substr(), mb_substr(), iconv_substr() and grapheme_substr() now consistently clamp out-of-bounds offsets to the string boundary. Previously, false was returned instead of the empty string in some cases.

According to PHP 8.0 migration guide, calls to these four functions should no longer trigger PHPStan errors on PHP >=8.

[dbrekelmans]

This should be resolved by #28 as it uses a safe 2.0 if you are running php8